The AASHTO standards for corrugated polyethylene pipe typically require the pipe be fabricated from HDPE. Current AASHTO standards require the polyethylene compositions comply with cell classification of 335400C according to ASTM D-3350. The cell classification of 335400C requires a maximum MI at 190 degrees Centigrade as per ASTM D1238 of 0.4 grams per ten minutes, a density of 0.945 to 0.955 grams per cubic inch as per ASTM D1505, minimum flexural modulus of 110,000 pounds per square inch according to ASTM D790 and minimum tensile strength of 3,000 pounds per square inch according to ASTM D638 and a minimum environmental stress crack resistance of 24 hours determined by a notched constant tensile load (NCTL) of 15% of the yield stress of the polyethylene tested as per ASTM D5397. These polyethylene compositions have an additional AASHTO requirement requiring the addition of at least 2% by weight of carbon black particles for ultra-violet resistance. Equally, important, the California State Department of Transportation (DOT) and AASHTO are considering requiring service life specifications for corrugated polyethylene pipe used for drainage as per test method for hydrostatic design basis for thermoplastic pipe, ASTM D2837. This means that existing polyethylene resins and blends of polyethylene resins are not capable of providing sufficiently enhanced ESCR. New grades of HDPE that are expected to be costly and/or new blends of commercially available polyethylene are required to satisfy the more stringent ESCR tests such as the Pent test, ASTM F1473, and test method for time-to-failure of plastic pipe under constant internal pressure, ASTM D1598.
Typically, corrugated polyethylene pipe manufacturers utilize specialty blow-molding grades of high-density polyethylene prepared in reactors by material suppliers and having bimodal or multi-modal molecular weight distributions. Debras et al. in U.S. Pat. No. 6,218,472 disclosed such a polyethylene composition which satisfies the current AASHTO standards and which is produced by means of a multi stage polymerization. The disadvantage of this approach is that the pipe manufacturer typically pays a premium for as polymerized virgin corrugated pipe grade high-density polyethylene and can not easily modify the physical properties of the polyethylene composition to enhance the physical properties or processability in relation to the pipe size and profile shape. Ideally, the corrugated pipe manufacturers would prefer to purchase lower cost prime (commodity polyethylene), wide and/or off specification virgin and/or post consumer and industrial recycled, reprocessed polyethylene components that they blend to meet the appropriate AASHTO standards.
Blending approaches have been disclosed. For example, Michie, Jr., U.S. Pat. No. 4,374,227, discloses medium density polyethylene pipe blends with improved low temperature brittleness properties and gloss which are composed of HDPE, LLDPE and a carbon black concentrate. Michie, Jr. discloses a thermoplastic Medium Density Polyethylene (MDPE) composition having a nominal density of 0.926 to 0.940 grams per cubic centimeter. Unfortunately, this approach has the disadvantage of too low a density to meet the cell classification of 335400C according to ASTM D-3350 for corrugated and profile HDPE pipe. Similarly Boehm et al. in their U.S. Pat. No. 5,338,589 and Morimoto et al. in their U.S. Pat. No. 5,189,106 disclose MDPE having density ranges of 0.930 to 0.940 grams per cubic centimeter. Boehm et al. and Morimoto et al. both utilize specific and different two-stage polymerization processes to produce blending components for the resulting medium density polyethylenes. The disadvantage of this approach is that it is limited to medium density polyethylene and excludes the high-density polyethylene density range of 0.945 to 0.955 grams per cubic centimeter required for corrugated and profile polyethylene pipe. Su in U.S. Pat. No. 4,824,912 discloses terblends of a major portion of LLDPE and minor amounts of HDPE of low molecular weight and of HDPE having high molecular weight. This approach also has the same disadvantage of being limited to low and medium density polyethylene compositions.
The object of this invention is to produce blends of commodity HDPE components that provide corrugated HDPE pipe compositions having a density range of about 0.951 to about 0.954 grams per cubic centimeter and MI in the range of about 0.15 to about 0.35 with ESCR in the range of about 24 to about 500 hours as measured by a NCTL ASTM D5397 procedure or equivalent range of ESCR values as measured by any other methods, for example, notched constant stress ligament (NCSL). Generally, commercially available HDPE copolymers polymerized to produce blow-molding grades of HDPE are often utilized for corrugated pipe. Several commercially available HDPE copolymer blow molding grades similar to Chevron Phillips 5202 HDPE grade comply with AASHTO standards for density, MI, flexural modulus and tensile strength but fail the environmental stress crack resistance (ESCR) requirements for NCTL ASTM D5397. The low ESCR is due to their characteristic broad molecular weight distribution (MWD) that includes low molecular weight fractions.
A further object of this invention is to disclose methods of selecting blend compositions of prime, wide and off specification and regrind virgin resins and post industrial and consumer recycled, reprocessed and regrind polyethylene resins that enhance ESCR of HDPE pipe blends by increasing the number of tie molecules between crystalline lamellae and thereby decreasing the number of molecular loose ends. The number of molecular loose ends is decreased by reducing number of shorter polyethylene molecules by melt blending polyethylene with sufficiently high molecular weight to provide exceedingly high ESCR with low molecular weight polyethylene components having narrow molecular weight distributions to provide improved processability. The methods disclosed in this invention are applicable to medium and high density polyethylene blend compositions having a density range of 0.94 to 0.954 grams per cubic centimeter. It is an additional object of this invention to disclose the specific molecular parameters required to select both the high molecular weight and the low molecular weight polyethylene components so that the number of loose ends associated with the short molecules are minimized and the physical properties of the blend composition meets the desired performance standards.
It is a further object of this invention to disclose lower cost HDPE compositions for corrugated plastic pipe than as polymerized polyethylenes having multimodal molecular weight distributions. In this regard, the invention discloses a method of varying the composition of high density polyethylene components having sufficiently different values of density and melt index such that the density and melt index of the blended composition can be varied independently to attain enhanced physical properties and processability respectively while maintaining an enhanced environmental stress crack resistance.
It is an additional object of this invention to provide HDPE pipe material with enhanced ESCR and long-term stress crack resistance by selecting a high molecular weight (HMW) HDPE component having a minimum value of the number average molecular weight so as to diminish the low molecular weight fraction of resulting blends with low molecular weight (LMW) HDPE components.
Enhanced physical properties such as flexural modulus and tensile strength are obtained by utilizing LMW HDPE homopolymer component having a characteristic narrow molecular weight distribution and higher density than the HMW HDPE component. Enhanced processability is obtained by utilizing low molecular weight HDPE copolymer component having a characteristic narrow molecular weight distribution devoid of short molecules and sufficiently high melt index to improve processability without dramatically greatly decreasing the ESCR.
It is the further objective of this invention is to provide the corrugated HDPE pipe and fittings manufacturers, the opportunity to vary the blend ratios of prime, wide and off specification and regrind virgin and post industrial and consumer recycled, reprocessed and regrind HMW and LMW HDPE's to obtain the required combination of physical and process properties of pipe and fittings. For example the pipe manufacturer may vary blend ratios to enhance 24-hour impact behavior of the pipe, ESCR and flexural stiffness by specific pipe diameter and corrugation design.
The invention provides the benefit of blending of prime, wide and off specification and regrind virgin and post industrial and consumer recycled, reprocessed and regrind HMW and LMW HDPE's to provide corrugated HDPE pipe and associated fittings and accessories material compositions having enhanced physical properties and processing characteristics that meet and exceed AASHTO standards.
The invention is described more fully in the following description of the preferred embodiment considered in view of the drawings in which: